Supervisory control system

ABSTRACT

A supervisory control system having three independent control links each capable of independently supplying a signal to direct a process being controlled. A first one of the control links embodies direct digital control; the second, automatic or semiautomatic analog control; and the third, manual control. The first control link includes an analog memory amplifier-receiving analog signals from a central control unit and supplying an output analog control signal. The second control link includes a known type analog controller controllable by an analog memory amplifier which is preset during direct digital control. The condition-indicating signal of the controlled process is supplied as the variable input to the controller such that transfer between the first link and the second link is &#39;&#39;&#39;&#39;bumples.&#39;&#39;&#39;&#39; The process variable signals are returned to a digital control element which integrates the variables in the system. The analog control signal from a digital control element is multiplexed to a plurality of process control stations, each of which has three independent controllers. Address selection is provided.

United States Patent [72] Inventor Anthony E. Turner Phoenix, Ariz. [21]Appl. No. 711,429 [22] Filed Mar. 7, 1968 [45} Patented Apr. 27, 1971[73] Assignee Motorola Inc.

Franklin Park, Ill.

[54] SUPERVISORY CONTROL SYSTEM 10 Claims, 3 Drawing Figs.

[52] U.S.C1 340/147, 318/20, 330/9, 330/51, 340/150 [51] lnt.Cl. H02h9/00, H03f 1/02,l-l031 1/14 [50] Field ofSearch 340/147, 150; 330/9, 10,51; 318/20.070, 20.395

[56] References Cited UNITED STATES PATENTS 3,290,563 12/1966 Hyer etal. 330/51UX 3,422,327 l/l969 McBrayer et al. 3 18/20.070UX 3,443,2355/1969 Newbold 330/51X Primary ExaminerDonald J. Yusko Attorney-Muellerand Aichele ABSTRACT: A supervisory control system having threeindependent control links each capable of independently supplying asignal to direct a process being controlled. A first one of the controllinks embodies direct digital control; the second, automatic orsemiautomatic analog control; and the third, manual control. The firstcontrol link includes an analog memory amplifier-receiving analogsignals from a central control unit and supplying an output analogcontrol signal. The second control link includes a known type analogcontroller controllable by an analog memory amplifier which is presetduring direct digital control. The condition-indicating signal of thecontrolled process is supplied as the variable input to the controllersuch that transfer between the first link and the second link isbumples. The process variable signals are returned to a digital controlelement which integrates the variables in the system. The analog controlsignal from a digital control element is multiplexed to a plurality ofprocess control stations, each of which has three independentcontrollers. Address selection is provided.

H K memu. eoumou. /59 lBIREfiL mew/n. CONTROL 25 16 11 l ELEMENT 51 G gSI 5 A i G Animist? snmou N5 |8 SELECTION I l MEA ARRAY L m I 20- i 21 II I aw I PROCESS l MEMORY CONTROLLING l 61 AMPLIFIER DEVICE I I Menus Il PROCESS 3 1 extra 1 I L -lti =i l6 PROCESS CONTROL srmon -J PROCESSCONDITION INDICATING SlGNALS SUPERVISORY CONTROL SYSTEM BACKGROUND OFTHE INVENTION This invention is related to that disclosed in anapplication of Lawrence R. Smith, Ser. No. 71 1,430 entitled SupervisoryControl System" filed Mar. 7, 1968 and assigned to the same assignee asthe subject application.

This invention relates to supervisory control system and particularly tothose systems capable of using direct digital control, analog control,or manual control.

Supervisory control systems can be characterized as being of two generaltypes. A first and common type is so-called incremental value system,while a second is a true-value system. Other terms for the incrementalvalue system are incremental algorithm or velocity algorithm. Anothername for the true-value system is true-position algorithm." In anincremental value system the control signals indicate to aprocesscontrolling device whether to increase, decrease, or make nochange. Such signals are repeated until the process-controlling devicehas changed to the desired position. Expressed in another way, theprocess-controlling device is the integrator in the system. That is, itsums the control signals to provide a true-control position. In atrue-value system, the controlling element generates signals indicatingthe desired or true value of the process-controlling device. Thesesignals are sent to the process-controlling device and it responds byadjusting itself to such desired value or position in accordance withits predetermined characteristics. In the true-value system theintegrating is accomplished in the controlling element rather than inthe process-controlling device. Such a control element may either beanalog or digital in character.

In emergency situations, an operator may have to assume manual controlof a supervisory control system. In the incremental type of controlsystem, the operator has to exercise control by means of a toggleswitch, for example, which would indicate to the process-controllingdevice to increase or decrease. Such control requires constant attentionby the operator. In a true-value system, operator intervention ischaracterized by the operator adjusting a dial to indicate in somemanner the desired true position of the process-controlling device. Itdoes not necessarily require operator attention at all times.

Various control elements for supervisory control systems are divided.into two broad categories, digital and analog. As used herein, the termanalog" means a supervisory control system in which the amplitude ofelectrical signals indicates a numerical value, while the term "digital"indicates that the presence or absence of signals, irrespective of theamplitude, indicatesarbitrarily selected numerical values. Such elementsmay be used in either the incremental value or true-value systemsdescribed above. A combination of digital and analog controllingelements are often found in supervisory control systems. For example,digital-controlling elements may generate a set of digital signals whichare then converted to an analog control signal. Such analog controlsignal is thenutilized as a set point for an analog controller whichcontrols the process-controlling device.

An example of a digital unit supplying signals to an analog controlleris described by I-I.S. Tsien ct al. in the American Rocket SocietyJournal, July 1952, pages 192 et seq. in an article entitle AutomaticNatigation of a Long Range Rocket Vehicle. The Tsien et al. system is atrue-value system. Similarly, a digital and analog combination for anincremental control system is described by Marson in an article entitledOutput and Standby Equipment for Direct Digital Control" in the MagazineCONTROL, page I37 et seq., Mar. I965, FIG. 2.

Digital equipments have many applications in supervisory control systemsand many advantages in that insofar as equipment variations areconcerned, they are drift free because the information or quantities arerepresented by discrete signals as opposed to analog signals whereinamplitudes represent values, which are subject to drift, and thereforeerror. If a digital equipment-controlled process is subjected to drift,the digital equipment when programmable can be reprogrammed or theprogram may be fixed.

Digital equipment, when used as a computational device, can by muchfaster than analog devices thereby a single digital control element,such as a computer, may be time shared by a plurality of analog controldevices. This time sharing of the digital control element can improveperformance of the system in that if it is programmed, any errors in thelogic design or algorithm can be corrected by merely changing theprogram rather than modifying equipments. For this reason and others, itis desirable to provide a digital control element for a supervisorycontrol system which controls a large plurality of remote processcontrol stations.

If an incremental value system were to be adopted, then the digitalcontrol unit computes each increment and supplies it to the analogcontroller. This arrangement is wasteful in that a lot of digitalcontrol element time is consumed in keeping track of the incrementalsystem performance. Digital control element time can be quite expensive.Therefore, in a supervisory control system having a central digitalcontrol element, it is desirable to adopt a true-value system. Further,in an incremental system with the integration being accomplished in ananalog device, such analog quantities are subject to drift due toequipment performance. In a true-value system, the

digital control element performs the integration and supplies adrift-free control signal.

It is unquestionable that in a true-value system the computationperformed in a digital control element will be the most accurate and themost stable. However, digital control elements, as well as connectingcontrol links, are subject to malfunctions. In controlling processes orother operations, it is desirable in several instances to continueoperations even through a digitalcontrol unit and its communication.network are not functioning, are functioning improperly, or have in somemanner created an error which wouldv be adverse to the successfulperformance of the process. Such continuation-should not have an adverseaffect on system operation.

A low cost process control system utilizing a central digital controlelement for a large plurality of process control stations, some of whichmay have a wide geographic distribution, requires a time sharing by thevarious stations of communication networks and the digital controlelement. Such time sharing requires a reliable memory capability at eachprocess control station for remembering the control signal. If digitalsignals are transferred to each station, then such memory capability maybe of the digital type. Additionally, each station requires adigital-to-analog converter to effect control over a process parameter.For a truly low cost process control system a single digital-to-analogconverter couldbe located at the central digitalcontrol element. Thensome system of multiplexing is required to supply a time-sharedcommunication network which caries a sequence of analogcontrol signalsto a plurality of process control stations.

SUMMARY OF THE INVENTION It is an object of the invention to provide animproved process control station with long term electrically alterableelectronic storage of analog control signals receivable on a time-sharedbasis.

It is another object of this invention to provide an improvedsupervisory controlsystem having a digital control element.

which has a.bumpless'backup system for assumingcontrol' of a controlprocess whenever the digital control unit fails.

It is a further object of this invention to provide bumpless transfer ofcontrol between a plurality of control links in. a control system.

A feature of this invention includes parallel control'links, one ofwhich is digital controlled and another of which automatically followsthe process perfonnance and is capable of assuming control of theprocess upon malfunction of the digital control unit without introducingunnecessary transients into the process.

Another feature of the invention is the provision of a supervisorycontrol system having an analog memory unit in a direct digital controllink and a similar analog memory unit in an analog-controller link withthe memory unit in the analog link automatically following the processcontrol signal variations as controlled by the direct digital link. Allthe controlling links can supply similar control signals indicative ofthe desired condition of the process.

Apparatus incorporating this invention includes a direct digital systemfor supervisory control when a first analog memory unit receives analoginput signals from a digital control element and stores them for anindefinite period of time. The analog memory amplifier continuouslysupplies an analog control signal indicated by the stored analog signalto a process-controlling device. An analog control link is providedwhich is connected to the process for receiving and automaticallytracking the continuously supplied control signal from the first controllink. A second analog memory amplifier continuously receives the processcondition indicating signals for storing same. Upon a malfunction in thedirect digital control link, or for other reasons, condition responsivemeans switch the control from the digital control link to the analogcontrol link. Input connections to the analog controller are switched tothe second analog memory amplifier. The set point input of the analogcontroller is then received from the second or set point'analog memoryamplifier while the control input is the process condition-indicatingsignal. In addition, a manual controller is a manual control link isprovided. Switching means select one of the three control links forcontrolling a processcontrolling device. All three of the control linksare in parallel system relationship insofar as the process-controllingdevice is concerned. A set point unit is connected in parallel with thesecond analog memory amplifier such that it may be used as an alternateset point source to the second analog memory amplifier.

A digital control element supplies digital-controlling signals to asingle digital-to-analog converter. An output analog signal from theconverter is supplied to a plurality of process control stations.Each'process control station receives such analog signals andselectively supplies them to the first analog memory amplifier in eachof the respective stations. The converter supplied analog signal is alsoselectively available to the second analog memory amplifier in each ofthe process control stations. The process condition-indicating signalsare supplied from every process control station to the digital controlelement. Each of the process control stations include gating means whichreceive station selection signals from the digital control element via astation selection array. These station selection signals open gates forselectively passing analog signals to the analog memory amplifiers inthe respective process control stations. In this manner, only one analogsignal line need be provided between the D/A converter and a pluralityof process control stations. Since a station-selection array is providedadjacent a digital ,control element, the number of lines between thecentral station and the plurality of remote stations is equal to twicethe square root of the number of remote stations.

THE DRAWING FIG. 1 is a block diagram illustrating a systemincorporating the teachings of the present invention and schematicallyshowing details of a novel process control station in the system.

FIG. 2 is a block-schematic diagram of an analog memory amplifier andits input gates usable with the FIG. 1 system.

FIG. 3 is a diagrammatic showing of the connection and support of ananalog memory element usable with the FIG. 2 memory DESCRIPTION OF THEILLUSTRATIVE EMBODIMENT signals. Cable 11 carries such digital processinput signals from digital control element 10 to digital-to-analogconverter 12. Converter 12 supplies corresponding analog input signalsover line 13 to a plurality of process control stations 14, 15, 16 and17. A much larger plurality of process control stations may be providedfor the system. Station 14 is illustrated in detail flow chart form; itbeing understood that the other stations l5, l6 and 17 may beconstructed in accordance with the showing of station 14. Depending upona process being controlled, the process-controlling device in therespective stations may be different or identical.

In order to reduce the number of wires between the central stationdigital control element and a large plurality of remote ly locatedprocess control stations, station-selection array 18 decodes digitaladdress signals received fromdigital control element 10 over cable 19.Station-selection array 18 consists of a decoder for decoding a largenumber of parallel-received signals into a smaller plurality of decodedsignals, as is well known in the digital and computer communicationarts. The decoded signals are supplied over cable 19a to all of theprocess control stations 14-17, two separate lines are connected to eachstation.

In station-selection array 18, for example, if there are remotestations; then the maximum number of lines between array 18 and theremote stations would be 100, i.e., one line for each remotely locatedprocess control station. By providing two address lines for eachstation, the number of stationselection lines between array 18 and thevarious stations is equal to twice the square root of the number ofstations, or 20 in this particular example. The trade-off for building asingle array or large plurality of station decoders versus the cost ofthe address signal lines is a matter of design choice.

As shown in FIG. 1, process control station 14 consists of three controllinks 20, 23 and 24. First control link 20 is utilized for directdigital control; that is, signals received from digital control element10 via digital-to-analog converter 12 directly controls theprocess-controlling device 21. A process condition-indicating signal issupplied over line 22 directly to the digital control element 10 as wellas to selected portions of the remote process control station 14, aswill be explained. Digital control element 10 has an analog digitalconverter (not shown) for digitizing the supplied condition-indicatingsignal. Second control link 23, also termed the analog control link, isutilized as a backup control link for direct sign digital control link20. (Iontrol link 23 can be operated either automatically orsemiautomatically. By backup to the direct digital link 20, it is meantthat second control link 23 assumes control of the process variablebeing affected by process-controlling device 21 whenever it is desiredor necessary to remove control of process-controlling device 21 formfrom first control link 20. As an example, a malfunction occurringwithin direct digital control link 20 may cause an automatic transfer ofcontrol to link 23. It should be observed that analog control link 23 isin parallel system relationship to direct digital control link 20.

Third control link 24 includes manual controller 75 for hard manualcontrol. Such a manual controller is used in the event of failure ormalfunction of either or both control links 20 and 23. Actuation andcontrol of process-controlling device 21 may also be transferred to thethird manual control link 24 at the discretion of the operator. In orderto provide bumpless transfer from manual control link 24 back toautomatic or semiautomatic control, second control link 23 automaticallytracks the operation of third control link 24 in the same manner as ittracks the control of process-controlling device 21 by direct digitalcontrol link 20. In the present illustration, the tracking of thecontrol is performed by sensing and storing the condition-indicatingsignal supplied over line 22 and thence over line 56 to control link 23.

Direct digital control link 20 includes analog memory amplifier 25having an input connection through analog input signal gate 26. Theanalog input signal supplied by digitaltoanalog converter l2 over line13 is continuously supplied to gate 26 of station 14 as well as tocorresponding gates in all of the illustrated process control stations-17. Gate 26 is selectively opened by station address signals appliedover line 28 to pass the appropriate analog signals reaching gate 26over line 27 within station 14. When digital-to-analog converter 12 issupplying an analog input signal on line 13 which is intended to bestored in an analog memory amplifier means 25, digital control element10 simultaneously supplies digital station selection signals to array 18which decodes same to supply a set of stations selection signals overline .28 for opening gate 26. As soon as analog memory amplifier meansreceives a new analog input signal, a corresponding changed analogcontrol signal is supplied over line 29 through selection switch toprocess-controlling device 21. The rate of change in the process islimited by the reaction of process-controlling device 21, it beingunderstood that the analog memory amplifier means 25 substantiallyinstantaneously supplies an analog control signal in accordance withreceived analog input signals. Of course, the condition-indicatingsignal on line 22 correspondingly changes and is stored within secondcontrol link 23 (through line 59) reflecting the change in controlaffected by digital control element 10. All process performance trackingfor backup is performed within the remote process control station.

Analog memory amplifier means 25 is illustrated in simplified schematicform in FIG. 2 with the capacitive element 38 of the memory amplifierbeing illustrated in diagrammatic form in FIG. 3. An analog input signalon line 27 is passed through first input gating stage 36 and thencethrough second input gating stage 37 to capacitive memory element 38.Input gating stages 36 and 37 each consists of an insulated-gatefield-effect transistor (IGFET). The gate electrodes of the IG- FETS arejoined to line 28A, the drain electrodes are respectively the inputconnections to the gating stages 36 and 37, the source electrodes arerespectively the output connections of gating stages 36 and 37 while thesubstrates are electrically unconnected. When gating stages 36 and 37are conductive, as caused by a signal on line 28A, the respective IGFETsprovide an extremely low impedance signal path between the source anddrain electrodes. Therefore the analog input signal is faithfullysupplied through gate 26 for storage in memory element 38.

Gate 26 is selectively opened when the station selection signals aresupplied over lines 28 to AND circuit 288 which combines them in a knownmanner into a signal digital gate control signal on line 28A. Forexample, a relatively positive signal on line 28A opens gate 26 while anegative signal closes the gate. The signal passed through gate 26 isstored in the capacitor 38 as an electric charge and is sensed by thevoltage magnitude appearing across the capacitor. The voltage magnitudeis supplied over line 39 to unity-gain output amplifier 40 which has anexceedingly high input impedance. Such input impedance is provided byconnecting the capacitive memory element 38 to a gate electrode of anIGFET. Output amplifier 40 supplies its output signal over line 29 toswitch 30 (FIG. 1). The signal on line 29 is also supplied throughvoltage divider 41, thence over feedback line 42 through resistor 43 tojunction 44 between the input gating stages 36 and 37. Output amplifier40 is preferably a unity-gain amplifier, such that when gate 26 isclosed (the IGFETS have a high impedance between source and drainelectrodes) the voltage on junction 44 is substantially equal to thevoltage on junction 45. This action minimizes leakage through inputgating stage 37. There are additional possible leakage paths to otherportions of the circuit. Leakage through these additional paths isminimized by a guard plane indicated in FIG. 2 by dashed-box 46.Metallic layer 47, FIG. 3, constitutes the guard plane of a physicalembodiment and is connected to feedback line 42 for receiving the outputsignal of line 29 which is substantially identical to the voltage atjunction 45. Therefore essentially zero voltage is provided across highquality insulating post 48 which practically eliminates leakage throughthat path. Further, if tubular capacitor 38 is pushed against thesupporting substrate 49 it will first contact metallic layer 47 reducingany possible leakage through the walls of the capacitor element 38.

Referring again to FIG. 1, analog control link 23 includes analogcontroller 50 of known design, having input signal means consisting of apair of differential input connections 51 and 52. Its output controlsignal is supplied over output signal means line 53 to terminal A ofswitch 30. As explained later, switch 30, shown as a manually actuatedsingle-pole triplethrow switch, is automatically actuated as indicatedby dotted line from sensing and condition-responsive means 54.Additional switches 55, 57 and 60 are ganged with switch 30 forsimultaneous actuation by condition-responsive means 54. When switch 30is set to terminal D the direct digital control link 20 controlsprocess-controlling device 21, when set to terminal A analog controllink 23 effects control over device 21 and when set to terminal M manualcontrol link 24 effects control over process-controlling device 21.

With the switches set as shown, direct digital control link 20 suppliesthe analog control signal over lines 29 and 31 to device 21. The analogcondition-indicating signal on line 22 is supplied over line 56 toterminal A of switch 55. The analog control signal on line 31 is fedback over line 32 to terminal D and terminal M of switch 57. Therefore,during direct digital control and during manual control the analogcontrol signal is supplied to-input terminal 51 of analog controller 50.In this manner analog controller 50 tracks the analog control signal.The signal output means line 53 of analog controller 50 is connected tothe D and M terminals of switch 55 which connects the output of thecontroller 50 to input connection 52. This completes theconnection foreffecting automatic tracking of the process being controlled to providea bumpless transfer from either digital control link 20 or frommanualcontrol link 24.

In addition to controller 50 automatically tracking the analog controlsignal supplied over line 32 when on direct digital control an inputsignal (condition-indicating signal) to set-point analog memoryamplifier means 58 as shown on line 59 is connected to both the D and Mterminals of switch 60. Gate 62 is electrically interposed between theoutput terminal of switch 60 and the input of set-point analog memorymeans 58. When so interposed, digital control element 10 mustperiodically send station selection address signals over line 63 toselectively open gate 62 for permitting the condition-indicating signalto be stored in set-point analog memory amplifier means 58. The outputof set-point analog memory amplifier 58 is supplied through switch 59 toterminal A of switch 57. Therefore, when condition-responsive means 54sets the respective switches to the A terminals, analog controller 50receives its set-point signal from set-point analog memory amplifier 58.Switch 55 being set to its A terminal passes the condition-indicatingsignal from line 56 to input 52; these connections are recognized as astandard connection to an analog controller from analog control forprocess control devices. Switch 60, on the input portion of set-pointanalog memory amplifier means 58, is set to terminal A for receiving asetpoint signal from the converter 12 via line 13 nd switch 61. In thelatter instance, gate 62 is selectively opened by digital controlelement 10 through the emission of station-selection address signals.

Additional versatility to the analog control link is provided byset-point unit 65. Unit 65 by may be manual or automatic. This unit canbe a slide wire unit automatically set by a servomotor or may be amanually adjusted set-point unit. Setpoint unit 65 supplies thisset-point signal over line 66 to switch 59. By resetting switch 59 tothe illustrated unconnected terminal, set-point unit 65 supplies aset-point signal to terminal A of switch 57. In this manner, when analogcontrol link 23 is selected, a predetermined set point is suppliedthrough analog controller 50 for controlling process device 21 when abumpless transfer is not mandatory. Also line 66 is connected to switch61 for selectively supplying its set-point signal to the terminal A ofswitch 60 for resetting the stored set point and analog memory amplifiermeans 58. With the above-described connections, set-point analog memoryamplifier means 58 initially stores the condition-indicating signal fromprocess-controlling device 21. Subsequent to switching control fromeither links 20 or 24 to analog control link 23, the operator may adjustset-point unit 65 to a predetermined value and then switch control ofthe process to set-point unit 65. Also, the operator may adjustset-point unit 65 to stored set-point signal of amplifier means 58 andthen manually adjust setpoint unit 65 to bring the process to adifferent state of operation, as desired.

Automatic transfer of control between the various independent controllinks may be effected by condition-responsive means 54. As shown, powersupply 77 supplies power through fuse 76 to independent control link 20.Upon a short circuit in an analog memory amplifier means 25, forexample, fuse 76 opens causing a large potential difference'betweenlines 78 and 79. This potential difference is sensed in a normal mannerby condition-responsive means 54 which resets switches 30, 55, 57 and 60simultaneously to the respective A terminals. It is understood thatcondition-responsive means 54 may be responsive to other parametersincluding process parameters for switching control from any one of thethree links to any other one of the three links. In switch 60, terminalsD, A, and M are termed link-selecting terminals.

Iclaim:

1. A process control station for effecting a control function in aprocess-controlling device by supplying a true-value analog controlsignal over an output line, a device supplying a condition-indicatingsignal,

the improvement including in combination,

a first control link connected to said output line and receiving saidtrue-value analog control signal for supplying the analog controlsignal,

a second control link having an electronic analog memoryamplifying meansand an analog controller with an input signal means and an output signalmeans,

said second control link receiving at least one of said signals andbeing responsive to said received signal to adjust said controller to becapable of independently supplying a backup analog control signalsubstantially identical to said analog control signal supplied by saidfirst link,

said input signal means consists of first and second input portions andsaid analog controller output signal means being connected to firstinput portion for supplying said backup analog control signal theretoand said output line being connected to said second input portion forsupplying said analog control signal thereto, and,

said condition-indicating signal being supplied to said electronicanalog memory amplifying means and said electronic analog-memoryamplifying means supplying a signal to said input signal means to adjustsaid analog controller to the condition of a process being controlled.

2. The process control station of claim 1 wherein said first controllink has an electrically alterable analog memory with amplifier meansfor supplying said analog control signals.

3. The process control station of claim 2 further including outputline-switching means for selectively connecting said output line to saidfirst control link for receiving said analog control signal or to saidsecond control link analog controller for receiving said backup controlsignal.

4. A process control station as set forth in claim 3 wherein said outputline-switching means includes condition-respon sive means operativelyassociated with said first control link and responsive to apredetermined condition in said first control link to actuate saidoutput line-switching means to switch to said second control link.

5. The process control station of claim 1 wherein said first controllink includes an electrically alterable electronic-type analog memoryamplifier for storing and supplying said analog control signal andhaving an input gating portion for receiving analog input signal toalter the stored signal,

a third control link having a manual controller supplying apredetermined analog control signal,

switching means electrically interposed between said output line of allof said control links for selectively connecting any one of said controllinks to said output line, and

further switching means in said second control link selectivelyinterconnecting said analog controller input signal means to saidelectronic analogmemory amplifying means and for disconnecting saidoutput line from said electronic analog-memory amplifying means.

6. The process control station of claim 1 wherein said first controllink has an electrically alterable electronic-type analog memoryamplifier for supplying said analog control signal and further includinginput gating means for receiving an analog input signal to be stored,

digital control element means including station-selection means anddigital analog converter means, said converter means and saidstation-selection means being connected to said input gating means forselectively supplying an analog input signal to said analog memoryamplifier of said first control link and said process-controlling devicesupplying said condition-indicating signal to said digital controlelement.

7. A process control system having a central station for monitoring andeffecting a control over a plurality of remote stations, each of saidremote stations having a process-controlling device responsive to ananalog control signal for effecting a function and supplying acondition-indicating signal indicative of the function being effected,

the improvement including in combination,

a digital control element in said central station operative to supplydigital control signals, a digital-to-analog signal converter means insaid central station responsive to said digital control signals tosupply an analog input signal indicative of said digital controlsignals, said converter supplying said analog signals to all of saidremote stations,

means in the digital control element receiving said condition-indicatingsignals from all of said remote stations,

station-selection array means in said central station for supplyingselection signals to said remote stations with each remote station beingresponsive to a unique set of stationselection signals to accept saidanalog input signal only when indicated by said station-selectionsignals,

each station including a plurality of independent control linksselectively connectable to an output control line for furnishing ananalog control signal to a process-controlling device,

' a first one of said control links being jointly responsive to ananalog signal supplied by said converter and to its unique set ofstation-selection signals to store said analog input signal and supplyan analog control signal substantially identical to said stored analoginput signal,

a second control link selectively jointly responsive to said analoginput signal and to said station-selection signals unique to therespective station for receiving and storing said analog input signaland selectively operative to supply an analog control signal to saidoutput line,

switching means electrically interposed between said first and secondones of said control links for selecting the analog control signal,

means in said second one of said control links for receiving and storingsaid condition-indicating signal and operative to supply a signalindicative of the condition of the process being controlled as an analogcontrol signal.

8. The system of claim 7 wherein each remote station further includes athird control link having a manual controller and said switching meansoperative selectively connect any one of said control links to saidoutput line.

9. The system of claim 7 wherein said first and second control linkseach have an electrically alterable electronictype analog memoryamplifier for storing and receiving said analog input signal and saidsecond control link having an analog controller selectively receivingsaid condition-indicating signal as a set point input and said analogcontrol signal of said second control link having variable input andwhen connected to said output line having switching means selectivelyconnecting said set point input to said analog memory amplifier in saidsecond control link and switching said condition-indicating signal to tosaid switching means for selecting one of said control links to supplyanalog control signal to the process-controlling device in accordancewith the sensed parameter.

1. A process control station for effecting a control function in aprocess-controlling device by supplying a true-value analog controlsignal over an output line, a device supplying a condition-indicatingsignal, the improvement including in combination, a first control linkconnected to said output line and receiving said true-value analogcontrol signal for supplying the analog control signal, a second controllink having an electronic analog memoryamplifying means and an analogcontroller with an input signal means and an output signal means, saidsecond control link receiving at least one of said signals and beingresponsive to said received signal to adjust said controller to becapable of independently supplying a backup analog control signalsubstantially identical to said analog control signal supplied by saidfirst link, said input signal means consists of first and second inputportions and said analog controller output signal means being connectedto first input portion for supplying said backup analog control signalthereto and said output line being connected to said second inputportion for supplying said analog control signal thereto, and, saidcondition-indicating signal being supplied to said electronic analogmemory amplifying means and said electronic analog-memory amplifyingmeans supplying a signal to said input signal means to adjust saidanalog controller to the condition of a process being controlled.
 2. Theprocess control station of claim 1 wherein said first control link hasan electrically alterable analog memory with amplifier means forsupplying said analog control signals.
 3. The process control station ofclaim 2 further including output line-switching means for selectivelyconnecting said output line to said first control link for receivingsaid analog control signal or to said second control link analogcontroller for receiving said backup control signal.
 4. A processcontrol station as set forth in claim 3 wherein said outputline-switching means includes condition-responsive means operativelyassociated with said first control link and responsive to apredetermined condition in said first control link to actuate saidoutput line-switching means to switch to said second control link. 5.The process control station of claim 1 wherein said first control linkincludes an electrically alterable electronic-type analog memoryamplifier for storing and supplying said analog control signal andhaving an input gating portion for receiving analog input signal toalter the stored signal, a third control link having a manual controllersupplying a predetermined analog control signal, switching meanselectrically interposed between said output line of all of said controllinks for selectively connecting any one of said control links to saidoutput line, and further switching means in said second control linkselectively interconnecting said analog controller input signal means tosaid electronic analog-memory amplifying means and for disconnectingsaid output line from said electronic analog-memory amplifying means. 6.THe process control station of claim 1 wherein said first control linkhas an electrically alterable electronic-type analog memory amplifierfor supplying said analog control signal and further including inputgating means for receiving an analog input signal to be stored, digitalcontrol element means including station-selection means and digitalanalog converter means, said converter means and said station-selectionmeans being connected to said input gating means for selectivelysupplying an analog input signal to said analog memory amplifier of saidfirst control link and said process-controlling device supplying saidcondition-indicating signal to said digital control element.
 7. Aprocess control system having a central station for monitoring andeffecting a control over a plurality of remote stations, each of saidremote stations having a process-controlling device responsive to ananalog control signal for effecting a function and supplying acondition-indicating signal indicative of the function being effected,the improvement including in combination, a digital control element insaid central station operative to supply digital control signals, adigital-to-analog signal converter means in said central stationresponsive to said digital control signals to supply an analog inputsignal indicative of said digital control signals, said convertersupplying said analog signals to all of said remote stations, means inthe digital control element receiving said condition-indicating signalsfrom all of said remote stations, station-selection array means in saidcentral station for supplying selection signals to said remote stationswith each remote station being responsive to a unique set ofstation-selection signals to accept said analog input signal only whenindicated by said station-selection signals, each station including aplurality of independent control links selectively connectable to anoutput control line for furnishing an analog control signal to aprocess-controlling device, a first one of said control links beingjointly responsive to an analog signal supplied by said converter and toits unique set of station-selection signals to store said analog inputsignal and supply an analog control signal substantially identical tosaid stored analog input signal, a second control link selectivelyjointly responsive to said analog input signal and to saidstation-selection signals unique to the respective station for receivingand storing said analog input signal and selectively operative to supplyan analog control signal to said output line, switching meanselectrically interposed between said first and second ones of saidcontrol links for selecting the analog control signal, means in saidsecond one of said control links for receiving and storing saidcondition-indicating signal and operative to supply a signal indicativeof the condition of the process being controlled as an analog controlsignal.
 8. The system of claim 7 wherein each remote station furtherincludes a third control link having a manual controller and saidswitching means operative selectively connect any one of said controllinks to said output line.
 9. The system of claim 7 wherein said firstand second control links each have an electrically alterableelectronic-type analog memory amplifier for storing and receiving saidanalog input signal and said second control link having an analogcontroller selectively receiving said condition-indicating signal as aset point input and said analog control signal of said second controllink having variable input and when connected to said output line havingswitching means selectively connecting said set point input to saidanalog memory amplifier in said second control link and switching saidcondition-indicating signal to said variable input and each of saidremote stations being operatively independent one on the other. 10.Subject matter of claim 9 further including condition-responsive meanssensiNg a parameter operatively connected to said switching means forselecting one of said control links to supply analog control signal tothe process-controlling device in accordance with the sensed parameter.